The memory devices are driven by the new application and the requirement of the future. The advance in the field of computer and communications will use large quantities of each species of memories. For example, computer interfaces will become to be operated by speech processing or vision processing, and other communication interfaces, all of which require a lot of memories. Memory technology will continue to move in the direction of increased numbers of devices in a wafer.
Typically, mask ROM includes at least two types of devices with difference threshold voltages in a wafer. A type of device is formed in an active area and another type of device with a threshold voltage mask formed in another active area during the process. For example, the first device is a normal device with threshold voltage V.sub.t1 and another device with a threshold voltage V.sub.t2, wherein the V.sub.t1 is different from V.sub.t2. Therefore, the second device needs an extra mask for ion implantation to obtain a different threshold voltage as provided with the first device. The method involves differentiating the threshold voltages by ion implantation of some of the transistor gates to differentiate the threshold voltage. This method raises the threshold voltage of the n-channel device by doping boron with heavy dose. This method is so called threshold voltage programming. In order to achieve above object, prior art obtains different threshold voltage with process that does not involve additional mask of the prior art for ion implantation. For example, in U.S. Pat. No. 5,506,438, Hsue proposes a method to form devices with different threshold voltage which does not include an extra masking step. There are another two methods for programming which are field oxide programming and through-hole programming. Each of the programming methods can be implemented at different stages of the process. The detailed schematic of a read only memory (ROM) and manufacturing process can be seen in "semiconductor devices" written by Betty Prince, John Wiley, Chapter 10, pp 507-512. The field oxide programming is introduced during the manufacturing process to differentiate the threshold voltage of the devices by using gate oxides with different thickness. The third one is to program when the contacts to the cells are opened by selectively opening the contact holes for each transistor to the drain.
The conventional method to form the mask ROM devices, at lease, one extra process is required to define a coded device, such as implant through a sacrifice oxide or polysilicon. The cycle time for the device coding by implanting through sacrifice oxide is too long, while implanting through the polysilicon, the gate oxide degradation is an issue.